Sex differences Flashcards
Development of sex organs
- Start with same gonads -> sexual differentiation -> different male/female sex organs
Factors determining the development of male sex organs
SRY region on Y chromosome codes for testis-determining factor that binds to DNA in cells of undifferentiated gonads and causes them to become testes (father sperm)
Testes produce hormones that have defeminising (anti Mullerian hormone) and masculinising (androgen) effects
Anti-Mullerian hormone
A peptide secreted by fetal testes
Has defeminising effects, inhibits development of Mullerian system
Androgens (2 of them)
Testosterone
acts on androgen receptors in cells of Wolffian system and stimulates development into male internal sex organs
Dihydrotestosterone produced FROM testosterone by 5alpha reductase
Critical to stimulate development into male genitals
Testis determining factor
230 amino-acid long protein
Point mutations can prevent development of testes
Factors determining the development of female sex organs
By default, primordial sex organs develop into female sex organs
In the absence of testis-determining factor primordial gonads develop into ovaries.
In the absence of androgens produced by testes, internal and external sex organs develop into female organs (without any other hormonal influences necessary).
Recent discovery about the development of female sex organs
Wolffian system regression requires COUP-TFII, a nuclear receptor, so is not a passive process (A Swain, 2017, Science 357:648).
The genetic sex of a human fetus is determined by
The father’s sperm.
The prenatal development of female internal sex organs requires
No hormones at all
Abnormal development of sex organs
XY sex reversal
Androgen insensitivity syndrome
Turner’s syndrome
Persistent Mullerian duct syndrome
XY sex reversal
Point mutations in the SRY region of the Y chromosome result in female sex organs in XY individuals (see Werner et al, 1995, Cell 81 :705).
Androgen insensitivity syndrome
A condition caused by congenital lack of functioning androgen receptors; in a person with XY sex chromosomes, causes development of a female with testes but no internal sex organs
Turner’s syndrome
The presence of only one sex chromosome (an X chromosome) results in lack of ovaries but otherwise normal female sex organs and genitalia
Persistent Mullerian duct syndrome
Congenital lack of anti-Mullerian hormone causes the development of both male and female internal sex organs in an XY individual
primary sex characteristics
Gonads
internal and external sex organs
Secondary sex characteristics
- enlarged breasts
- widened hip
- facial hair
- deep voice
- sexual maturity
are not developed until puberty
Puberty is triggered by…
hypothalamic secretion of gonadotropin-releasing hormone (GnRH).
Organisational hypothesis
Based on findings that exposing female rats and guinea pigs to androgen in utero during critical periods altered their adult sexual behaviour
William Young and colleagues proposed that early androgens, similar to their effects on sex organs, organise the developing CNS in a masculine way, so as to make:
female behaviours less likely (defeminisation)
male behaviours more likely (masculinisation)
Without effects of androgens, the animals would behave in a female way – that is, as the sex organs, the animal’s brain and behaviour would by default develop in a female way
Neural and behavioural sexual dimorphisms
In line with the organisational hypothesis many neural sexual dimorphisms in rodents have been demonstrated to depend on organising effects of androgens during critical developmental periods
Other neural sexual dimorphisms have been demonstrated to depend on both organising and activating effects of androgens, and yet others only involve activating (and reversible) effects during adulthood.
There is evidence that neural sexual dimorphisms contribute substantially to sex differences in behaviour
There is also evidence for an interaction between environmental and hormonal influences on neural and behavioural sexual dimorphisms – so ‘nature’ and ‘nurture’ seem to interact.
effects of estrogen
Curiously, it was found in many cases that estrogens are as effective as testosterone in masculinising brain and behaviour in rodents.
Moreover, many masculinising effects of testosterone were mediated by estrogen receptors.
Aromatisation hypothesis
Due to estrogen findings some CNS cells testosterone is converted to estrogen by an enzyme called aromatase before it acts on estrogen receptors to exert masculinising effects
Protection hypothesis
The brains of developing rodents are ‘protected’ from the indiscriminate masculinising action of estrogen by an estrogen-binding protein, alpha-fetoprotein, in the blood. Testosterone is not bound by the protein, so can enter CNS cells where it can be converted to estrogen and then exert its masculinising effects.
Hypothesis is strongly supported by finding that alpha-fetoprotein mutant mice show masculinised brains and behaviour (Bakker et al., 2006, Nature Neurosci 9:220).
Aromatisation in rodents
There is definitive experimental evidence that aromatisation plays a role in masculinising effects of testosterone in rodents. However, if aromatisation also plays a role in other species is not known.
Organisational effects occur only during…
Critical periods
Determinants of sex differences in behaviour and cognition
Sex genes, activity of different sex hormones, and environment/experience are all factors that differ between males and female
Sex differences in behaviour may be partly mediated by sexual dimorphisms in the brain or CNS
Needed for heterosexual mating
Attractivity
Proceptivity (willingness to mate)
Receptivity (ability to mate)
Phases of mating
Phase 1
Attractivity
Proceptivity (willingness to mate)
Receptivity (ability to mate)
Phase 2
Appetitive
Phase 3
Consummatory
Phase 4
Post Consummatory
Spinal nucleus of the bulbocavernosus (SNB)
Collection of motor neurons in the lower lumbar spinal chord; controls the bulbocavernosus muscle at the base of the penis.
These motoneurons and muscles are necessary for normal penile reflexes that are important for successful copulation (Monaghan & Breedlove, 1992, Brain Res 587:178)
- They are absent or substantially reduced in size/number in adult females as compared to males.
Nature vs nurture rat mother
Rat mothers are stimulated to lick their male pups more often than their female pups because of testosterone in urine.
Such anogenital licking contributes to normal male sexual behaviour in the adult and to a normal number of SNB neurons.
posterodorsal medial amygdala (MePD)
MePD volume and cell size depend on testosterone action in adulthood
Sexually dimorphic nucleus of the preoptic area (SDN-POA)
SDN-POA is masculinised by testosterone during a critical perinatal period
Fliers & Swaab, 1985, Science 228:1112
One nucleus in the POA of hypothalamus was larger in volume and cell number in males than in females.
Hence, authors named the nucleus SDN.
Allen et al, 1989, (J. Neurosci. 9: 497)
- Studied four nuclei in the POA, which they named interstitial nuclei of the anterior hypothalamus (INAH) 1-4.
- INAH1 corresponded to SDN of Fliers&Swaab (1985), but did not differ between sexes. INAH4 also did not differ.
- INAH2 and 3 were larger in men than in females.
LeVay, 1991, Science 253:1034
- Found no significant sex differences in INAH1,2, and 4.
- Replicated that INAH3 was larger in heterosexual men than in women.
- Found additionally that INAH3 did not differ between homosexual men and heterosexual women.
Mating behaviour and relevant neural sexual dimorphisms - conclusions
Sexual dimorphisms exist in CNS regions that have been implicated in sexually dimorphic mating behaviour
This is consistent with the idea that neural sexual dimorphisms may contribute to behavioural sexual dimorphisms
in line with the organisational hypothesis, some aspects of sexually dimorphic mating behaviour and relevant neural sexual dimorphisms have been shown (in rodents) to involve organising effects of sex steroids during critical developmental periods
Sex differences in aggressive behaviour
While aggression is not a unitary concept, some aspects of aggression are strongly related to reproductive behaviour (e.g., competition for mating partner, protection of offspring).
Aspects of aggression are mediated by brain regions that overlap with regions implicated in reproductive behaviour; they are sex dependent and under the influence of sex steroids (there is particularly strong evidence for a role of testosterone).
In humans, there is evidence for some aspects of aggression being sexually dimorphic
Sex differences in cognitive functions
Women show superior performance in tasks requiring perceptual speed, verbal fluency, visual memory and fine motor skills
Men tend to do better than women on spatial tasks
Kimura 1992
Male advantage in…
Place learning and navigation
(Astur et al., 1998) Watermaze
correlate with a larger hippocampus (a structure that is critical for spatial learning) in males (Jacobs et al., 1990)
Affective disorders (with the exception of mania) and anxiety disorders
more prevalent in women
(Table, Kessler et al., 1994)
substance abuse disorders and antisocial personality disorder
more prevalent in men
(Table, Kessler et al., 1994)
Autism-spectrum disorders
more prevalent in males than in females (mean ratio ca. 4:1)
